Conventionally, in a wireless communication system, to which Long Term Evolution (LTE) is applied, a communication control procedure, called closed-loop precoding, is used for a down link (DL). A down link is a line directed from a base station (eNB: eNodeB) to a mobile station (UE: user equipment). In closed-loop precoding, a base station forms directional beams by using a plurality of antennas (that is, multiple antennas). Further, in closed-loop precoding, spatial multiplexing, by which data streams are simultaneously transmitted, may be used together with rank adaptation, by which the number (transmission rank) of the spatially multiplexed data streams (spatial layer) is adaptively controlled. The mobile station selects the optimum precoding matrix from a precoding codebook prescribed for each rank and feeds back (reports) the selection to the base station. The precoding codebook includes at least one precoding matrix.
Specifically, in closed-loop precoding, the mobile station feeds back channel state information (CSI) indicating a channel state to the base station. The CSI includes a rank indicator (RI) indicating a recommended communication rank (transmission rank), a precoding matrix indicator (PMI) indicating a recommended precoding matrix, and a channel quality indicator (CQI) indicating a wireless channel quality in a case where the RI and PMI are assumed. When the CSI is received from the mobile station, the base station determines, based on the RI and PMI, a precoding matrix. The base station then transmits, to the mobile station, reference signals specific to each mobile station (that is, UE-specific reference signals (RS)) and a shared channel (for example, a physical downlink shared channel (PDSCH)), to which the determined precoding matrix has been applied. The mobile station demodulates the PDSCH by using a channel estimate value based on the UE-specific RS.
As described above, a conventional method of feeding back a CSI supposes PDSCH transmission, to which single user-multiple input multiple output (SU-MIMO) is applied, the SU-MIMO being a spatial multiplexing technique for a signal to a single mobile station. That is, it is supposed that reliability is ensured by retransmission control. Therefore, the base station considers transmission efficiency for the single mobile station to be important, and selects a rank corresponding to the wireless channel quality and a precoding matrix corresponding to the rank.
Further, for LTE, a technique called three-dimensional MIMO (3D-MIMO) has started to be studied. In 3D-MIMO, antennas that are two-dimensionally arranged, that is, multiple antennas arranged in a two-dimensional array, are used. In 3D-MIMO, by use of the antennas two-dimensionally arranged, directional beams in a horizontal direction and directional beams in a vertical direction are formed. Several methods of utilizing the directional beams in the horizontal direction and the directional beams in the vertical direction have been proposed. For example, a utilization method has been proposed, in which a conventional fixed sector in a horizontal direction is adaptively divided in an elevation angle direction, by use of a directional beam in a vertical direction. According to this proposal, by dividing a cell into many sectors, communication capacity of the whole system is able to be increased. Further, a utilization method has been proposed, in which a signal is transmitted, by use of a directional beam in a vertical direction, to each of mobile stations that are present at different heights, for example, to each of mobile stations that are present on different floors in a building. By this utilization method, communication characteristics of each of the mobile stations are improved, and interference among communication of the different mobile stations is able to be alleviated.
Non-Patent Document 1: IEEE 802.16 Broadband Wireless Access Working Group Closed Loop MIMO Precoding (Apr. 11, 2004)
Non-Patent Document 2: 3GPP TSG-RAN WG1, R1-130302, “Discussion on scenarios for elevation beamforming and FD-MIMO,” January 2012
Non-Patent Document 3: 3GPP TSG-RAN, RP-122034, “Study on 3D-channel model for Elevation Beamforming and FD-MIMO studies for LTE,” December 2012
In recent years, a technique has been developed, which is for dynamically switching over between the above described SU-MIMO and multiple user-multiple input multiple output (MU-MIMO), which is a spatial multiplexing technique for a signal to a plurality of mobile stations. Further, a technique is in the process of being developed, which is for applying closed-loop precoding to an enhanced physical downlink control channel (EPDCCH), which is a control channel compatible with frequency multiplexing, in addition to conventional time multiplexing. Therefore, in order to be compatible with these techniques, enhancement of a method of feeding back CSI from a mobile station to a base station has been desired.
As a new method feeding back CSI, a method of feeding back multiple CSI processes has been proposed, for example. In this method of feeding back multiple CSI processes, limitation related to a subset of codebooks is added for each CSI process. A mobile station feeds back CSI from a range of RI and PMI limited according to a bitmap indicated by an upper layer. In an application example, a mobile station feeds back CSI supposing SU-MIMO of a recommended rank of a connected cell in a CSI process 1. Further, the mobile station feeds back CSI supposing MU-MIMO and an EPDCCH of a rank 1 of the connected cell in a CSI process 2.
In this manner, by the above described method of feeding back CSI, system performance is improved. However, for a single cell, the mobile station transmits CSI of a size (for example, 12 bits) that is twice the size for that of the conventional one, to the base station. Accordingly, overhead of control information transmitted to the base station when the mobile station feeds back the CSI is largely increased.
Moreover, when 3D-MIMO is introduced, the size of the CSI may be increased further, and the overhead of the control information may be largely increased even further.